I have toyed with this idea many years and never did this based on the resulting image issues, but the central ribbon or even a 3/4" ceramic coated dome idea should result in a better focused image.
Hooking one up with a simple 6th cap and L pad would prove interesting.
One thing about the quads that make them special is the incredible downward dynamic range that eclipses most other electrostatics. While they have limited overall dynamics they make up for it going the other direction.
That stack must surely cure any issues on dynamics!
Did you series or parallel these?

The Quads were run in parallel. We should have tried series connection (on the 300B) but didn't.

I think the sounds stage issue is a matter of setup and distance to the speakers. If you have enough distance then, theoretically, there should not be too much difference between a single pair and a double pair. Unfortunately we had a busy program and lots of people wanted to audition the stacked Quads, so once they were set up there was little time to play around and experiment.

At my place I don't have the space to put them up. I'm going to do a series of measurements on a mono stack and probably sell the frames after that.

I've been browsing where I normally don't and came across this thread and thought a zombie post was in order.

I've been running stacked quads for a while now, powered from a rebuilt ST70 with KT66s. My room is nowhere near as big as the one shown, and I tend to sit fairly close to the panels. I have found that positioning is very important - an inch or 2 is all it takes to throw it off. Probably more fussy than a single set.

I use a nordost set up disc that I picked up at a show. It has a useful set of tones that pan up and across the stereo image - they are very useful for working out distance and toe in. The other thing that I think is very important is the rake - I have found that having the panels dead vertical sounds best - I've tried everything from the "standard" angle to "falling forward" and vertical works very well for my room at least. This can be awkward to try out, but its worth it.

One of my sets is from 1974 (never serviced afaik) and the other set are very early, from 1959, serviced by quad in 1981. I know they are probably not at their best now but can't bring myself to tear them down to service them! Foolish maybe...

Arend-jan - what was the chipamp you mentioned in the post? I tried running them from an F5 but that combination didn't err, get on. Burn out resistors in the F5!

I've been thinking about this and I think set-up becomes more difficult when you are in a smaller room and close to the speakers. It was not as critical as I expected it to be in this large room. In fact it was easier than a single pair in a small room. But I must admit I did not fiddle with it endlessly to get the most out of them, when it was good enough it was good enough.

When you say having the panels dead vertical, do you mean you removed the panels from their original frames and mounted them flat in a custom frame?

The amplifier that we used was the Tentlabs b-amp that comes to the market this summer. Very nice indeed!

You might well be right on the setup in a smaller room. I also have an issue with so boominess in that room that was greatly reduced by the careful positioning. On the whole though, the stacked pair sound better than a single set.

No, I haven't removed the panels from the speaker, just that the whole speaker is vertical in the frame - and the frame is also vertical. I originally built my frames with a rake of about 10deg off vertical, but later experiments led me to change to vertical.

I haven't tried wiring them in series - must try that now, plus I must try my lm3875 chipamp too...

Fooling around stacked Quads from as way back as the 70's. Stacked quads need room, If you are using a small room , be prepared to treat the room Diffusers is a necessity . Here's a pic of a friends recent setup , the room is full of diffusers and it is actually a pretty big room..

Ah sure no-one would have those taps available unless you did some kind of custom build. The sound improved a bit with the series winding (sounded better on 16R taps rather than 8R too), so it must be a bit easier on the amp/better match. Going to do some more experiments tomorrow....